Essential oils contain terpenes, which are responsible for lots of pleasant plant odors and lots of medically beneficial things:

One thing that’s obvious about cannabis is that it has a strong smell. Actually, it can have lots of different smells: skunk, berries, pine, fuel, lemon, black licorice, Girl Scout cookies…Those smells come from terpenes that are Generally Recognized As Safe, because they’re found in things we eat all the time, like pepper. The wide variety of smells means that plants can differ a lot in terpene content. Some of these terpenes are arousing. Some are relaxing. Some reduce pain and inflammation. Some may have anti-addictive properties (selective CB2 agonists). That is to say, it’s actually true that different strains of marijuana are better suited for different uses, or different times of day.

Common use of “sativa” for stimulating and “indica” for sedating is apparently wrong, though.

Terpenes also mean there’s probably something beyond psychosomatic about the “contact high” phenomenon. If you can smell the weed, and the smell of weed is itself a mix of drugs…

Smoking cannabis is literally a form of aromatherapy that compensates for our unnaturally low exposure to plant smells. We evolved in the presence of those smells, and they seem to help things go better.